In a destacker which comprises a plurality of attractor means having vacuum cups or magnets etc. and which are mounted on the body frame in such a manner that they can be moved up and down freely, the attractor means serving to attract sheet-shaped blank materials from the stack one by one, and conveyor means having attracting function for conveying the blank materials to a working apparatus such as a press machine etc., there is a tendency of occurrence of two sheets or more of blank materials being attracted at one time by the attractor means due to the attracting action of oil deposited thereon; that is; so-called "double-blank" phenomenon. If and when the double blanks are conveyed or supplied to the press machine as they are, the metal molds thereof is damaged. Accordingly, it is necessary to detect the double blanks and prevent them from being conveyed to the press machine as they are.
In the prior art double-blank detecting apparatus as disclosed, for example, in Japanese Patent Publication No. SHO54-12671 (AIDA ENGINEERING CO., LTD), there is known an apparatus for detecting the thickness of the blanks by means of a contact type sensor in the course of conveyance or the like.
In such a prior art double-blank detecting apparatus, it is essential to ensure that the contact type sensor is brought into contact with the blank being conveyed. However, the sensor mounting condition in the prior art double-blank detecting apparatus has the following disadvantage. For example, in case of detecting the double-blank in the course of lifting the blank by means of an attractor, a detection error tends to occur because the attractor means and the blank are moving upwards when the detection is made, and further there may occur an error in the contact of the sensor with the blank incidental to the vertical movement of the attractor means thus making it impossible to ensure that the sensor is brought into contact with the blank. Moreover, in case blanks are detected in the course of conveyance of them to a press machine, in order to ensure that the detection is made in a limited space, it is essential to stop the conveyance temporarily and detect the double-blank phenomenon. This will cause a problem on lowering of the working efficiency.